Surgically implantable knee prosthesis

ABSTRACT

A prosthesis is provided for implantation into a knee joint compartment between a femoral condyle and its corresponding tibial plateau without requiring bone resection. The prosthesis includes a body having a generally elliptical shape in plan and a pair of opposed surfaces where one of the surfaces is generally concave. The body further includes an exterior portion and an interior portion, where the exterior portion is constructed from a higher modulus material than the interior portion such that the body is at least slightly deformable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention pertains to a prosthetic device which issurgically implantable into a body joint, and more particularly to aknee joint prosthesis which may be surgically implanted between thefemoral condyle and tibial plateau of the knee compartment.

2. Background Art

Articular cartilage and meniscal cartilage provide the mobile weightbearing surfaces of the knee joint. Damage to these surfaces isgenerally due to genetic predisposition, trauma, and/or aging. Theresult is usually the development of chondromalacia, thinning andsoftening of the articular cartilage, and degenerative tearing of themeniscal cartilage. Various methods of treatment are available to treatthese disease processes. Each option usually has specific indicationsand is accompanied by a list of benefits and deficiencies that may becompared to other options.

The healthy knee joint has a balanced amount of joint cartilage acrossthe four surfaces of this bicompartmental joint (medial femoral condyle,medial tibial plateau, lateral femoral condyle, and lateral tibialplateau). In patients with osteoarthritis, the degenerative processtypically leads to an asymmetric wear pattern that leaves onecompartment with significantly less articular cartilage covering theweight bearing areas of the tibia and femur than the other compartment.Most commonly, the medial compartment of the knee joint is affected moreoften than the lateral compartment.

As the disease progresses, large amounts of articular cartilage are wornaway. Due to the asymmetric nature of the erosion, the alignment of themechanical axis of rotation of the femur relative to the tibia becomestilted down towards the compartment which is suffering the majority ofthe erosion. The result is a varus (bow-legged) deformity in the case ofa medial compartment disease predominance, or a valgus (knock-kneed)deformity in the case of lateral compartment disease predominance.Factors such as excessive body weight, previous traumatic injury, kneeinstability, the absence of the meniscus, and genetic predisposition allaffect the rate of the disease.

Osteoarthritis is usually defined in stages of Grade I through V, withGrade III revealing significant articular cartilage loss, Grade IVrevealing some eburnation of the subchondral bone, and Grade V detailingboth significant articular loss and bone loss. The disease manifestsitself as periodic to continuous pain that can be quite uncomfortablefor the patient. The cause of this pain is subject to many opinions butit is apparent that, as the joint compartment collapses, the collateralligament on the side of the predominant disease becomes increasinglyslack and the tibial and femoral axes move, for example, from a varus toa valgus condition. This increases the stress on the opposing collateralligament as well as the cruciate ligaments, and shifts the load bearingfunction of this bicompartmental joint increasingly towards the diseasedside. This increasing joint laxity is suspected of causing some of thepain one feels. In addition, as the bearing loads are shifted, the bodyresponds to the increased loading on the diseased compartment with anincreased production of bony surface area (osteophytes) in an attempt toreduce the area unit loading. All of this shifting of the knee componentgeometry causes a misalignment of the mechanical axis of the joint. Thismisalignment causes an increase in the rate of degenerative change tothe diseased joint surfaces, causing an ever-increasing amount ofcartilage debris to build up in the joint, and further causing jointinflammation and subsequent pain.

Currently, there is a void in options used to treat the relatively youngpatient with moderate to severe chondromalacia involving mainly onecompartment of the knee. Current treatments include NSAIDS, cortisoneinjections, hyaluronic acid (HA) injections, and arthroscopicdebridement. Some patients cannot tolerate or do not want the risk ofpotential side effects of NSAIDS. Repeated cortisone injections actuallyweaken articular cartilage after a long period of time. HA has shownpromising results, but is only a short term solution for pain.Arthroscopic debridement alone frequently does not provide long lastingrelief of symptoms.

Unfortunately, the lack of long term success of these treatments leadsto more invasive treatment methods. Osteochondral allografts andmicrofracture techniques are indicated for small cartilage defects thatare typically the result of trauma. These procedures are not suitablefor addressing large areas of degeneration. In addition, osteochondralallografts can only be used to address defects on the femoral condyle,as tibial degeneration cannot be addressed with this technique. Hightibial osteotomy (HTO) corrects the varus malalignment between the tibiaand the femur but, because it is performed below the joint line, it doesnot fill the cartilage void or re-tension the medial collateral ligament(MCL). Removing bone and changing the joint line does not complicate theconversion to total knee arthroscopy (TKA). However, an HTO does leave ahard sclerotic region of bone which is difficult to penetrate, makingconversion to a total knee replacement (TKR) technically challenging.Unicompartmental and bicompartmental total knee replacements resectsignificant amounts of bone and, if performed on younger patients, willlikely require revision surgery as they age. Revision total kneereplacement surgery is usually extensive and results in predictablydiminished mechanical life expectancy. Therefore, it is best to delaythis type of bone resecting surgery as long as possible.

The only true solution is to rebuild the defective joint by “filling”the joint space with more articular bearing material through a completeresurfacing of the existing femoral condyle and tibial plateau. Byreplacing the original cartilage to its pre-diseased depth, the jointmechanical axis alignment is restored to its original condition.Unfortunately, these natural articular materials and surgical technologyrequired to accomplish this replacement task do not yet exist.

Currently, replacement of the existing surfaces, with materials otherthan articular cartilage, is only possible with a total or uni-condylarknee replacement, and these procedures require removal of significantamounts of the underlying bone structure. The alternative method is tofill the joint space with a spacer that replaces the missing articularmaterials.

Attaching a new bearing surface to the femoral condyle is technicallychallenging and was first attempted, with limited success, over 40 yearsago with the MGH (Massachusetts General Hospital) knee. Like a dentalcrown, it covered both the femoral condyles with Vitallium (CoCr) andwould bear against the existing tibial plateau. Tibial covering devicessuch as the McKeever, Macintosh, and Townley tibial tray maintained theexisting femoral surface as the bearing surface but, like the MGH knee,all required significant bone resection, thus making them less thanideal solutions as well. These devices also made no particular attemptto match the patient's specific femoral or tibial geometry, thusreducing their chances for optimal success. Because these devices weremade of CoCr, which has different viscoelastic and wear properties fromthe natural articular materials, any surface geometry which did notclosely match the bearing surface of the tibia or femur could causepremature wear of the remaining cartilage due to asymmetric loading.

Newer materials technologies in development including filling the jointspace by injecting polyurethane (U.S. Pat. No. 5,795,353) into the jointand anchoring it with holes drilled into the tibial plateau. Othersinclude a series of polymeric materials such as PVA hydrogels in atitanium mesh (see Chang et al, Journal of Biomedical Materials Research37, 51-59, 1997), biodegradable anhydride prepolymers that can becross-linked with irradiation by UV light (U.S. Pat. No. 5,902,599), andin vivo grown articular chondrocytes in a collagen fiber or otherbiocompatible scaffold (U.S. Pat. No. 5,158,574). Other low surfaceenergy materials, such as low temperature isotropic (LTI) pyroliticcarbon, have been investigated as bearing surfaces as well. However,these techniques are limited by one's ability to first of all fashionthese materials in a conformal manner to replicate the existing kneegeometry, while at the same time maintaining their location within thejoint, while further being able to survive the mechanical loadingconditions of the knee.

U.S. Pat. Nos. 6,206,927 and 6,558,421 and copending U.S. applicationSer. No. 10/232,608, each of which are incorporated by reference herein,disclose a prosthesis for the knee compartment which fills the jointspace in order to replace the missing articular materials. Thisprosthesis provides an anatomically correct bearing surface for both thetibial plateau and femoral condyle to articulate against. Additionally,the prosthesis reduces the concentrated loads on the femoral condyle andits articular cartilage and maintains proper spatial location of thefemoral condyle to the tibial plateau, thereby restoring normal jointalignment. Advantageously, the prosthesis does not require any boneresection or any means of bone fixation.

In addition to these benefits, it is also desired to provide aunicompartmental prosthesis which has improved load absorbingcharacteristics, provides improved load carrying ability throughout thecomplete range of motion (ROM), and increases patient comfort.

SUMMARY OF THE INVENTION

Accordingly, a prosthesis is provided for implantation into a bodyjoint, such as the knee joint compartment between a femoral condyle andits corresponding tibial plateau, without requiring bone resection. Theprosthesis includes a body having a generally elliptical shape in planand a pair of opposed surfaces where one of the surfaces is generallyconcave. The body further includes an exterior portion and an interiorportion, where the exterior portion is constructed from a higher modulusmaterial than the interior portion, such that the body is at leastslightly deformable.

According to one embodiment of the present invention, the interiorportion is hollow. According to another embodiment, the interior portionis preferably constructed from materials such as reinforced andnon-reinforced elastomeric polymers, viscous-elastic materials, andhydrogels. The exterior portion is preferably constructed from materialssuch as ceramics, metals, metal alloys, hydrogels, and reinforced andnon-reinforced thermoset or thermoplastic polymers, or compositesthereof. Either the exterior portion or the interior portion can beconstructed at least partially from a polymer capable of containingliving cells, and can include an active material associated therewith.The exterior portion can include a surface coating, such as for reducingfriction of the prosthesis.

A locking mechanism can be disposed in the interior portion and arrangedto connect the pair of opposed surfaces to enhance the integrity of theprosthesis. In a preferred embodiment, the locking mechanism includes afirst mating component affixed to one of the opposed surfaces and asecond mating component affixed to the other opposed surface andarranged to be securely received by the first mating component. Thefirst and second mating components can preferably move relative to oneanother.

In further accordance with a preferred embodiment of the invention, thepair of opposed surfaces includes a top surface that is generallyconcave and a bottom surface that is generally flat. A peripheral edgeextending between the top and bottom surfaces and includes a first side,a second side opposite the first side, a first end, and a second endopposite the first end. A first dimension D is defined by the first endand the second end, and a second dimension F is defined by the firstside and the second side, wherein the dimension F is from about 0.25D toabout 1.5D. Both the top and bottom surfaces are preferably contouredsuch that the prosthesis is self-centering within the knee jointcompartment.

According to the present invention, outside edges along a periphery ofthe body are rounded. A periphery of the body is on average of greaterthickness than a central portion of the body, and preferably a sectionacross the body from the first end to the second end generally has theshape of a negative meniscus. In a preferred embodiment, a sectionacross the body from the first side to the second side has a thicknessat a periphery of the first side which is larger on average than athickness at a periphery of the second side. The body is preferably freeof any means of fixation within the knee joint compartment, but canalternatively be arranged to be constrained within the knee jointcompartment.

Correspondingly, a method is provided for implantation of a prosthesisinto a knee joint compartment between a femoral condyle and itscorresponding tibial plateau without requiring bone resection. Themethod includes providing a prosthesis including a body having agenerally elliptical shape in plan and a pair of opposed surfaces whereone of the surfaces is generally concave, the body having an exteriorportion and an interior portion wherein the exterior portion isconstructed from a higher modulus material than the interior portion.The method further includes surgically exposing the knee jointcompartment and inserting the prosthesis into the knee jointcompartment.

In further accordance with the present invention, the method can furtherinclude determining a size and shape of the prosthesis required byexamination of the knee joint, where the examination can include X-rayimaging or MRI imaging. The prosthesis can be selected from a library ofprostheses of standard shapes and sizes, or alternatively a customprosthesis can be generated where the size and shape are at leastpartially based on the examination of the knee joint. If necessary, thecondition of tissue located in the knee joint compartment or in adjacentareas can be altered, and an active material can be associated with thebody. The prosthesis is preferably pre-loaded during implantation.

In further accordance with the present invention, a method is providedfor correcting misalignment in an axis of rotation of a knee joint. Themethod includes providing a prosthesis including a body having agenerally elliptical shape in plan and a pair of opposed surfaceswherein one of the surfaces is generally concave, the body having anexterior portion and an interior portion wherein the exterior portion isconstructed from a higher modulus material than the interior portion.The method further includes surgically exposing the knee joint, andinserting the prosthesis into the knee joint to at least partiallycorrect the misalignment of the axis of rotation. The method can includeinserting the prosthesis into a medial compartment of the knee joint andmoving the axis to a less varus condition, or can include inserting theprosthesis into a lateral compartment of the knee joint and moving theaxis to a less valgus condition.

The above features and advantages, along with other features andadvantages of the present invention are readily apparent from thefollowing detailed description of the best mode for carrying out theinvention when taken in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view illustrating an implantable knee prosthesisaccording to the present invention;

FIG. 2 is a top plan view of the prosthesis of FIG. 1;

FIG. 3 is a side elevational view of the prosthesis of FIG. 1;

FIG. 4 is an end elevational view of the prosthesis of FIG. 1;

FIG. 5 is a top plan view of the prosthesis according to the presentinvention with reference to a coordinate system;

FIG. 6 is a cross-sectional view of a first embodiment of the prosthesistaken along line A-A of FIG. 5;

FIG. 7 is a cross-sectional view of a first embodiment of the prosthesistaken along line B-B of FIG. 5;

FIG. 8 is a cross-sectional view of a second embodiment of theprosthesis taken along line A-A of FIG. 5;

FIG. 9 is a cross-sectional view of a third embodiment of the prosthesistaken along line A-A of FIG. 5; and

FIG. 10 illustrates an exemplary placement of the prosthesis accordingto the present invention in a knee joint.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The prosthesis according to the present invention is designed to besurgically implantable into a body joint to replace damaged tissuetherein. More particularly, the prosthesis of the present invention is aunicompartmental device suitable for minimally invasive, surgicalimplantation into a knee compartment without requiring bone resection.The knee compartment is defined by the space between a femoral condyleand the respective tibial plateau, in which a portion of the naturalmeniscus is ordinarily located. By effectively replacing worn articularmaterial, the prosthesis of the present invention restores the normaljoint alignment and provides a smooth bearing surface against which thefemoral condyle can articulate. Degeneration of the femoral anatomy issignificantly reduced because the conforming femoral surface of theprosthesis accommodates the complex shape of the femoral condyle inextension as well as in flexion. Further, it essentially eliminatesarticulation of the femoral condyle against the tibial plateau, therebypreventing further degradation of the tibial surface. By occupying thejoint space and retensioning the collateral ligaments, the prosthesisaccording to the present invention improves joint stability and restoresthe limb to a more normal mechanical alignment.

By the term “unicompartmental” it is meant that each prosthesisaccording to the present invention is suitable for implantation into butone medial or lateral compartment defined by the space between a femoralcondyle and its associated tibial plateau. In other words, the presentprosthesis is not a bicompartmental prosthesis which, in one rigiddevice, could be inserted into both of the two femoral condyle/tibialplateau compartments. In many, if not most, cases a prosthesis will beinserted into one compartment only, either the medial or lateralcompartment. Most often, it will be the medial compartment as themeniscus and associated articular surfaces in the medial compartment aremost subject to wear and damage. Additionally, it is possible to inserttwo separate prostheses into the medial and lateral compartments of thesame knee, or to use two such prostheses that are mechanically, butnon-rigidly, linked. Advantageously, the prosthesis according to thepresent invention functions to at least partially correct misalignmentin the knee axis of rotation due to disease. More specifically, whenplaced in the medial compartment, the prosthesis moves the knee axis 300(see FIG. 10) into a less varus, more valgus condition (typically 0-5°valgus). Likewise, when placed in the lateral compartment, theprosthesis moves the knee axis 300 into a less valgus condition.

The prosthesis according to the present invention is preferablytranslatable but self-centering. By “translatable” it is meant thatduring natural articulation of the knee joint the prosthesis is allowedto move or change its position, such that articulation of the kneeresults in a modest amount of lateral/medial and anterior/posteriortranslation of the prosthesis relative to the tibial plateau. Thus, thepresent prosthesis preferably is devoid of means of physical attachmentwhich limit its movement, for example, screws, mating ridges anddepressions, porous areas to accommodate tissue regrowth, and the like.

The femoral condyle has two major anterior/posterior radii such that,when the knee is in full extension, one radius position is in contactwith the tibial plateau while, during flexion, another portion of thefemoral condyle is in contact with the tibial plateau. In addition, thefemur rotates with respect to the tibia during flexion, thereby changingthe orientation of the femoral anatomy to the tibial plateau. The term“self-centering” means that upon translation from a first position to asecond position during knee articulation, the prosthesis of the presentinvention will return to substantially its original position as thearticulation of the knee joint is reversed and the original kneeposition is reached. Thus, the prosthesis will not progressively “creep”towards one side of the compartment in which it is located, but ratherthe prosthesis is shaped such that the contour of the prosthesis and thenatural articulation of the knee exerts a restoring force on thefree-floating prosthesis. The angle of attack of the femoral condyleand/or tibial plateau bearing surfaces against the prosthesis willensure that the prosthesis reversibly translates during articulation,maintaining the prosthesis, on average, in the same location for anygiven degree of knee articulation. The centered, rest position, of theprosthesis is usually determined when the knee is in extension and thereis maximum contact between the femoral condyle and the prosthesis. Inorder to ensure the ability of the prosthesis to “self-center,” adequatetension of the cruciate ligaments should be maintained.

While the prosthesis according to the present invention is shown anddescribed herein as being implanted in a knee joint, it is understoodthat the prosthesis could be utilized in joints other than the knee,such as the hip, shoulder, wrist, ankle, or elbow.

Turning now to FIGS. 1-4, an implantable knee prosthesis 100 accordingto the present invention is illustrated. Prosthesis 100 includes a body102 having a generally elliptical shape in plan and including a bottom,or tibial, surface 104 and an opposite top, or femoral, surface 106.Bottom surface 104 is preferably generally flat and top surface 106 ispreferably generally concave. However, it is understood that othercontours of top and bottom surfaces 104, 106 are fully contemplated inaccordance with the present invention. For example, depending on thecondition of the ligaments and other soft tissue structure at the timeof surgery and how much stability the knee will require, for a medialcompartment implantation top surface 106 typically ranges from generallyflat to concave and bottom surface 104 typically ranges from generallyflat to convex. For a lateral compartment implantation, top surface 106can range from generally convex to generally concave and bottom surface104 typically ranges from generally flat to concave. It is alsounderstood that the terms “concave” and “convex” as used herein are notrestricted to describing surfaces with a constant radius of curvature,but rather are used to denote the general appearance of the surfaces.

With continued reference to FIGS. 1-4, body 102 further includes aperipheral edge 112 extending between bottom surface 104 and top surface106 and having a first side 114, a second side 116 opposite first side114, a first end 118, and a second end 120 opposite first end 118. Asshown, edges along the periphery of prosthesis 100 are rounded ratherthan presenting sharp corners, such as in those devices of U.S. Pat. No.5,158,574. This rounded periphery is desired due to the fact thatprosthesis 100 is preferably allowed to move within the joint cavity.Periphery 108 of body 102 is on average of greater thickness than acentral portion 110 of body 102 (see FIG. 1), and preferably body 102generally has a negative meniscus shape when viewed from the side (seeFIG. 3) or in a section across body 102 in an anterior-posteriordirection from first end 118 to second end 120 (see, for example, FIG.6). Furthermore, in a preferred embodiment, a section across the body ina medial-lateral direction from first side 114 to second side 116 has athickness at a periphery of first side 114 which is larger on averagethan a thickness at a periphery of second side 116 (see FIG. 7).

In the embodiment depicted herein, second side 116 of body 102 includesa pair of lobes 122 a and 122 b, where lobe 122 a is adjacent first end118 and lobe 122 b is adjacent second end 120, with an indentation 124formed therebetween. When implanted in a patient's knee compartment (seeFIG. 10), indentation 124 will be proximate to the tibial spine and canpreferably be designed to vary in shape from patient to patient asnecessary due to the great range of variability of human anatomy. Withindentation 124, prosthesis 100 is generally kidney-shaped when viewedin plan, with the shape resembling a distorted ellipse. Of course,indentation 124 is not required, and other variations of bodyconfiguration are fully contemplated according to the present invention.Accordingly, it is understood that the term “generally elliptical” isintended to include all construction methods which yield a generallyplanar shape which is longer in one direction than in the transversedirection and has rounded corners, and that prosthesis 100 is nototherwise limited to any particular shape.

FIGS. 6 and 8-9 show an anterior/posterior cross-sectional view ofprosthesis 100 taken along section line A-A of FIGS. 2 and 5. Similarly,FIG. 7 illustrates a medial/lateral cross-sectional view of prosthesis100 taken along section line B-B of FIGS. 2 and 5. As shown in FIGS. 6and 7, prosthesis 100 includes an exterior portion 130 and an interiorportion 132. Exterior portion 130, which includes bottom and topsurfaces 104, 106, comprises a relatively high modulus material ascompared with interior portion 132, wherein the material is alsopreferably low friction. Materials suitable for the construction ofexterior portion 130 include, for example, metals such as steel ortitanium, metal alloys such as those described in U.S. Pat. Nos.3,989,517; 5,368,659; and 5,618,359 (LiquidMetal, Inc.), ceramics,hydrogels, and reinforced and non-reinforced thermoset or thermoplasticpolymers. The hardness of the material for exterior portion 130 shouldbe sufficient to span defects in the tibia or femur withoutsubstantially deforming into the defects, allowing for the provision ofrecessed or non-contacting areas of the prosthesis to encouragearticular regeneration.

Exterior portion 130 need not be made only of a single material, butcomposite structures may be used. Other possible materials are thosewhich can replicate the function of naturally occurring cartilage ormeniscus such as the CSTI meniscal repair material that is the subjectof numerous U.S. patents to Stone, for example, U.S. Pat. No. 5,158,574.A surface coating on exterior portion 130 can be utilized, such as forthe reduction of friction of prosthesis 100. Generally, the areas ofexterior portion 130 expected to have the most wear due to either highstress or greater movement relative to the femoral condyle or tibialplateau may be made of stronger, more abrasion resistant material thanthe remainder of prosthesis 100 when composite structures are used.However, it is understood that any single component may be softer thanthe material used for constructing the majority of exterior portion 130.

In contrast, interior portion 132 comprises a lower modulus material ascompared with exterior portion 130, where interior portion 132 is atleast slightly deformable in order to absorb load energy. Depending uponthe specific materials used for exterior portion 130 and interiorportion 132, the relative thicknesses and proportions of exteriorportion 130 and interior portion 132 are chosen to allow thisdeformability, but are not otherwise limited to any specificconfiguration. In a preferred embodiment, the ratio of interior portionthickness to exterior portion thickness is about 10:1. With reference toFIG. 8, suitable material choices for interior portion 132 includehydrogels, elastomeric polymers such as nylon, silicone, polyurethane,polypropylene, polyester, or the like, optionally fiber-reinforced,viscous-elastic materials, as well as other hydrophilic materials orhydrophobic materials. Alternatively, interior portion 132 can be hollowas depicted in FIGS. 6 and 7.

The hard yet deformable nature of prosthesis 100 of the presentinvention advantageously accommodates both the conformal fit that amatched component offers in an ideal knee and the mismatch that oftenoccurs when one introduces patient specific kinematic ROM such asfemoral roll-back and differing amounts of medial pivot or lateralpivot-shift mechanisms into the “real world” equation. The intactmeniscus carries 50-90% of the total load in a medial knee joint, andprosthesis 100 of the present invention accomplishes load absorptionthrough deformation as well as movement. Importantly, prosthesis 100 canmimic the conformal meniscal behavior not just in extension, butthroughout the range of motion, thereby reducing wear and load on theremaining articular surfaces and increasing patient comfort.

Prosthesis 100 of the present invention can include a locking mechanism140, such as that shown in the cross-sectional view of FIG. 9, whichinterlocks the bottom and top surfaces 104, 106 together to maintain theintegrity of the prosthesis 100, such as in the hollow embodiment ofFIGS. 6 and 7 or in case of breakdown of the load absorbing material(see FIG. 8) in interior portion 132. Preferably, locking mechanism 140comprises a first mating component 142 affixed to an interior side ofbottom surface 104, and a second mating component 144 affixed to aninterior side of top surface 106 and arranged to be securely received byfirst mating component 142. Locking mechanism 140 is preferablyconstructed of the same material as exterior portion 130 and moldedintegrally therewith. With this configuration, components 142, 144 oflocking mechanism 140 can move relative to one another to allowdeformation of prosthesis 100 during use, yet still limit the maximumspacing of surfaces 104, 106. Of course, it is understood that lockingmechanism 140 and its components 142, 144 could have a differentstructure other than that depicted herein. Furthermore, lockingmechanism 140 could alternatively comprise a solid connection betweenbottom and top surfaces 104, 106.

In accordance with the present invention, prosthesis 100 may bemanufactured so as to substantially contain, or have deposited thereon,a biologically or pharmaceutically active material such as, for example,one that promotes tissue regrowth, retards tissue degeneration, ordecreases inflammation. This is particularly suitable when prosthesis100 functions to bridge a defective area of bone or articular cartilage.The active material can be provided in the form of a coating on exteriorportion 130, or can be contained within exterior portion 130 or interiorportion 132 in the form of a solid, liquid, gel, paste, or soft polymermaterial. Such active materials may be designed to be delivered at onceor in a timed-release manner. Preferably, the area of prosthesis 100containing the active material does not actually contact, or minimallycontacts, the damaged tissue. In addition, exterior portion 130 orinterior portion 132 can be constructed of a material, such as abiocompatible polymer, capable of containing living cells.

As stated above, one purpose of the prosthesis 100 of the presentinvention is to achieve a span-like effect to bridge areas of thefemoral condyle and/or tibial plateau which have been damaged or haveexperienced tissue degeneration. If too soft and/or low modulus of amaterial were to be used for the entire prosthesis as in prior artdevices, not only would the load not be concentrated on healthy tissue,but damaged areas would also be subjected to static and dynamic loadingand wear, thereby decreasing the opportunity for the body's naturalregenerative capability to function. Under such circumstances, activematerials will be rapidly dissipated and newly regenerated articularcartilage not having the necessary density or cohesiveness to withstandwear will quickly be eroded away. Rather than substantially deforming asin prior art devices to distribute a load relatively equally on themating femoral and tibial surfaces, prosthesis 100 according to thepresent invention does not necessarily spread the load uniformly, butrather may redistribute the load to healthy tissue, spanning areas ofimperfection and allowing inflamed, diseased, or other damaged areas toregenerate. Moreover, as regeneration proceeds, the regenerating tissuewill assume a shape dictated by the shape of prosthesis 100. Growthunder these circumstances has the greatest potential for dense, orderedcartilage most closely replicating the original surface.

Prosthesis 100 is preferably allowed to be mobile, accommodating a widevariety of patient kinematic types. However, the forces required todeform the prosthesis 100 must be balanced with the forces attempting tomove the prosthesis 100 from its position on the tibia. These forces arepresent regardless of whether the prosthesis 100 is fixed to the tibiaor not. Thus, while prosthesis 100 is described herein as being mobile,it is fully contemplated that prosthesis 100 could easily be convertedto an embodiment in which its movement is constrained by attachment tothe tibia, the surrounding synovial membrane, meniscal remnants, or thelike. In this embodiment, the intent is not necessarily to fixprosthesis 100 in one permanent location, but rather to limit its motionin order to balance motion of prosthesis 100 with deformation and loadabsorption and also as a means of preventing dislocation.

Much study has been dedicated to determine if any relationship exists inthe normal human anatomy that would allow one to define the requireddimensions of the prosthesis for proper fit and function based on asingle, easy to establish, measurable anatomic landmark. Based on astudy of over 100 MRI's and 75 X-rays of human subjects ranging from 15to 87 years of age, a relationship was established between theanteroposterior radius of the most distal portion of the femoral condyleand the dimensions which control the geometric form of the prosthesis.The database revealed a range of femoral anteroposterior radii from 32mm to 48 mm. However, it is known that the worldwide range is muchlarger because of genetic differences in the human anatomy.

With reference now to FIG. 5, a preferred method of construction of theprosthesis 100 of the present invention aligns the apex of a femoralradius with the Coordinate System Origin (CSO) 10. The apex of a tibialsurface is also generally aligned in both the anterior/posterior withthe CSO 10, but is separated vertically from the CSO 10 to create thepart thickness. The substantially elliptical shape of peripheral edge112 is then located with respect to the CSO 10. In general, the CSO 10of the prosthesis 100 is located at the center of the ellipse. It hasbeen found that a suitable size for body 102 is defined by a minor axisof the ellipse F (defined by first side 114 and second side 116) and amajor axis D (defined by first end 118 and second end 120) which arerelated by a ratio ranging from F=0.25D to 1.5D. Similar ratios can beestablished for all of the controlling dimensions of the part such thatthe shape in plan, femoral surface geometry, and tibial surface geometryfor a normal tibial anatomy can generally be defined by one physicalanterior/posterior measurement of the patient's tibial anatomy. Theappropriate thickness of the prosthesis 100 can be determined bymeasuring the amount of joint space between the femoral and tibialsurface when a minor amount of valgus (heels out, knees in) is appliedto the knee.

Referring to FIGS. 5-7, the preferred relationship between femoralradius RA (see FIG. 6) to other joint dimensions (femoral radius is thedriving relationship to all other dimensions) is as follows:

-   -   Medial-lateral radius RB=0.25RA to 1.0RA (see FIG. 7)    -   Curve of anterior half of femoral radius RC=0.5RA to 2.0RA,        posterior half is straight    -   Length D=0.6RA to 1.4RA    -   Posterior half E=0.1RA to 0.75RA    -   Width F=0.25RA to 1.5RA    -   Width from part center to medial edge G=0.096RA to 0.48RA    -   Anterior plan radius RH=0.16RA to 0.64RA    -   Posterior plan radius RM=0.16RA to 0.64RA    -   Radius along lateral spine area RP=0.1RA to 2.0RA    -   Width from part center to lateral edge Q=−0.32RA to 0.32RA    -   Location of transition from anterior radius to medial radius        Y=−0.32RA to 0.32RA

(A negative value means that a dimension may extend to an opposite sideof section line A-A).

The actual shape of the prosthesis 100 may be tailored to theindividual. Individuals with high varus or valgus deformation due towear, degeneration, or disease may require a prosthesis which is ofconsiderably greater thickness over the portions where wear is mostadvanced. In youthful patients, where trauma-induced damage rather thansevere wear or degeneration has occurred, differences in prosthesisthickness will be more moderate.

The axis of rotation of the tibia on the femur is 90 degrees to the pathof the tibial plateau against the femoral condyle. The two tibialplateaus (medial and lateral) are not in the same plane with each otherbut do act in a relatively constant radius to their respective femoralcondyles. In other words, although the symmetry of the femoral side ofthe prosthesis may be matched with the femoral condyle while the leg isin full extension, the rotation of the tibial plateau against thefemoral condyle is along a constant axis of rotation (90 degrees to theaxis of rotation), thus the angularity of the axis of symmetry of thefemoral condyle relative to the axis of symmetry of the tibial plateauis not parallel but at some acute angle. Also, the axis of symmetry ofthe tibial plateau is not parallel to the path of rotation of the tibiarelative to the femur, but also at some mildly acute angle. Thus, thetrue orientation of the prosthesis, regardless of the relativeorientations of symmetry of the tibial side to the femoral side is 90degrees to the true axis of rotation as described in Hollister et al.,“The Axes of Rotation of the Knee,” Clin Orthopaedics and Rel Res 290,pp. 259-268, 1993, herein incorporated by reference. Any localizedpositions of higher loads are self-limiting due to the ability of theprosthesis to translate both rotationally and laterally which mimics thetrue motion of the natural meniscus as described by Hollister.

During the load bearing portion of the gait cycle, or stance phase,flexion at the knee does not typically exceed 35°. Thus, the highestcompressive loads in the knee occur with the knee substantiallyextended. The outer contours of the prosthesis are therefore preferablydesigned to substantially mate with the corresponding tibial and femoralsurfaces when the knee is in full extension so that the high compressiveloads can be distributed over large surface areas. The contact areasbetween the femoral condyle and the femoral surface of the prosthesisand between the tibial plateau and the tibial surface of the prosthesisare substantially equivalent during extension. However, because thecontour of the femoral surface is more concave, the femoral condyledetermines the position of the prosthesis on the surface of the tibialplateau in extension.

As the knee is flexed, the mating along the tibial surface issubstantially maintained. However, the contoured mating surfaces of thefemoral condyle and femoral surfaces of the prosthesis of the presentinvention can become increasingly dissimilar when the joint articulates.As the knee is flexed, there is a relative external rotation andposterior translation of the femur with respect to the tibia. Thus, thecontour angle of the femur becomes more in-line with the contour angleof the tibia in flexion. This can cause relative lateral or rotationalmovement, in the tibial plane, between the femoral condyle and thefemoral surface of the prosthesis. The forces generated by theincreasingly different geometry creates a rotational moment in thetibial plane which is resisted along the mating tibial surfaces andwhich also results in a restoring force tending to correctly locate theprosthesis along the femoral condyle. Thus, the prosthesis isself-centering to the femoral condyle, in part as a result of theconformity between the femoral condyle and the femoral surface of theprosthesis.

By changing the femoral surface of the prosthesis, it is possible toreduce the rotational moment induced during flexion by the mismatchbetween the femoral surface of the prosthesis and the femoral condyle. Apreferred method to accommodate this motion is to have a less acutealignment between the femoral and tibial axes of symmetry posterior tothe anterior/posterior midline, thereby reducing the mismatch betweenthe two axes in flexion. This angle is preferably 0° and can range from+/−15°. Anterior to the midline, the femoral contour is bent around aradius RC that is tangent to the posterior section of the sweep plane atthe most distal point of the femoral anterior/posterior radius RA. Thisfemoral surface geometry is essentially a compromise between thedifferent extension and flexion alignments of the femoral and tibialaxes of symmetry.

Because the prosthesis 100 according to the present invention preferablyhas no physical method of attachment, the generally concave femoralsurface serves to locate the prosthesis through all ranges of motion. Ofcourse, it is understood that proper tensioning of the collateralligaments is also important to maintain positioning of the prosthesis.By the very nature of the ability to adjust for the lost articularmaterial through the thickness of the prosthesis, the thicknessadjustment substantially eliminates the need for a functional meniscusas a bearing surface in a severely (Grade III or IV) degenerated knee.In these instances, the femoral surface of the prosthesis residessignificantly above the meniscal edge, and the meniscus is completelyunloaded.

The prosthesis according to the present invention also increases thetranslational stability of the knee. The conforming shape of the femoralsurface limits excessive anterior to posterior translation of the femur.As a result, this prosthesis possibly eliminates the need for ACLreconstruction in the older patient.

Generally speaking, each knee presents a different geometry of therespective femoral condyles and tibial plateaus. Even with respect tothe right and left knees of a single individual, although bilateralsymmetry dictates that the left and right knee components should bemirror images, this is often only an approximation. Thus, the shape ofthe affected femoral condyle and tibial plateau (while discussed hereinin the singular, more than one pair of condyle(s)/plateau(s) may beinvolved), will have to be ascertained to determine the correct geometryof the prosthesis 100 for a given patient.

To implant a prosthesis that possesses the characteristics required bythe subject invention, the patient's knee joint may be examined by anon-invasive imaging procedure capable of generating sufficientinformation such that one appropriately sized and shaped device may beselected. A variety of non-invasive imaging devices may be suitable, forexample magnetic resonance imaging (MRI), X-ray devices and the like.

Two methods of non-invasive imaging for selection of a suitableprosthesis 100 are preferred. In the first method, MRI or othernon-invasive imaging scans, optionally coupled with exteriormeasurements of the dimensions of the relevant tibial and femoralportions including the surface of the articular cartilage of the tibiaand femur, may be used to establish a library of prostheses whose sizeand geometry differ according to the age and size of the patient, thepatient's genetic make-up, and the like. A limited number of “standard”prostheses are then made to meet the requirements of a genericpopulation of patients. In this first method, a non-invasive imagingscan, such as an X-ray or MRI, together with knowledge of the patient'sgenetic make-up, general body type, extent of the disease, degeneration,or trauma and the like, will enable the surgeon to select a prosthesis100 of the correct size and shape from the library for the patient.

In a second method, each patient receives one or more prostheses thatare custom tailored for the individual by producing a contour plot ofthe femoral and tibial mating surfaces and the size of the meniscalcavity. Such a contour plot may be constructed from imaging data (i.e.,X-ray or MRI data) by a suitable computer program. From the contourplot, the correct surface geometry of the prosthesis 100 is determinedfrom the shape of the respective tibial plateau and femoral condyle andthe orientation between the two surfaces in extension. In general, theshapes just mentioned also include the articular cartilage which istypically maintained substantially intact.

In accordance with the present invention, it has been discovered thatthe amount of varus deformity is the primary, non-invasive method fordetermining the necessary thickness of the prosthesis 100 required forproper functioning. Viewing a weight bearing anteroposterior X-ray, acut and paste of the line drawn through the femoral condyles andrepositioned to put them once again parallel to the tibial plateaus willyield a measurement for the approximate thickness of the prosthesis.However, typically the proper thickness of the prosthesis is determinedintra-operatively.

Insertion of the prosthesis 100 of the present invention is typicallydone via a 3 cm to 5 cm medial parapatella incision. The prosthesis 100is introduced by arthroscopically assisted implantation, generallylimited to extensive clean-up of existing damaged tissue, e.g., torn orparticulate natural meniscus damage, osteophyte resection, etc. Thenatural meniscus may be maintained in position or may be wholly orpartially removed, depending upon its condition. Under ordinarycircumstances, pieces of the natural meniscus which have been torn awayare removed, and damaged areas may be trimmed as necessary. In somewhatrarer instances, the entire portion of the meniscus residing in themeniscal cavity may be removed or is not present. No bone resection ormechanical fixation of the prosthesis 100 is required. Only osteophyteswhich interfere with the prosthesis placement or with proper collateralligament alignment are removed.

Prosthesis 100 may also be used in conjunction with ACL or PCL repair,tibial osteotomy or articular surfacing procedures such as cartilagetransplantations or abrasion anthroplasty. Following insertion of theprosthesis, X-ray, fluoroscopy, or MRI may be used to assess the correctpositioning of the prosthesis both intraoperatively as well aspostoperatively. Since the surgical procedures used are not severe, andalso not irreversible, an unsuitable prosthesis may be readily removedand replaced, either with a different prosthesis from the library, or bya custom prosthesis.

Prosthesis 100 of the present invention is preferably pre-loaded in acompressed state during implantation. Thus, if the joint is temporarilyunloaded during periods of exercise, the expandability of the prosthesisof the present invention advantageously allows the prosthesis tomaintain contact with both condylar and tibial surfaces throughout theROM.

FIG. 10 illustrates prosthesis 100 positioned in a right knee joint 200between a femur 202, including the femoral condyles 204, and a tibia 206including the tibial plateau 208. The femur 202 and tibia 206 includeinterconnecting collateral ligaments 210. FIG. 10 illustrates theposition of first side 114, second side 116, first end 118, and secondend 120 of prosthesis 100 when inserted in the medial compartment of apatient' right knee joint 200. Of course, prosthesis 100 according tothe present invention may just as easily be implanted in a lateralcompartment or in the left knee of a patient.

One preferred surgical procedure which may be used to implant theprosthesis 100 according to the present invention can be described bythe following steps:

-   -   1. Verify preoperative indications:        -   a. Valgus determination of <5′ with erect anterior/posterior            X-ray;        -   b. Medial compartment disease only. Some lateral spurs may            be present; and        -   c. Pre-operative sizing via medial/lateral template            measurement of anterior/posterior X-ray.    -   2. Standard Arthroscopy surgical prep:        -   a. Infiltrate knee with Lidocaine/Marcaine and Epinephrine.    -   3. Arthroscopy:        -   a. Inspect lateral patello-femoral compartments for            integrity, some mild arthosis is acceptable;        -   b. Removal of medial meniscus toward the rim along the            anterior, medial and posterior portions;        -   c. Initial arthroscopic osteophyte removal via ⅛″ osteotome            and burr to allow for valgus positioning of the knee;        -   d. Complete the removal (to the rim) of the posterior and            posterior-lateral meniscus; and        -   e. Confirm sizing of the prosthesis by measuring distance            from resected meniscus to remaining anterior meniscus.    -   4. Medial parapatellar arthrotomy (mid-patella to tibial joint        line).    -   5. Complete removal of visible osteophytes along the medial        femoral condyle.    -   6. Insert thickness gauge and size for thickness of prosthesis.    -   7. Insert trial component:        -   a. Flex knee to approximately 50+degrees to fully expose the            distal portion of the femoral condyle;        -   b. Insert trial component; and        -   c. While applying insertion pressure, apply valgus stress to            the tibia and “stretch-extend” the tibia over the trial            component.    -   8. Check for proper sizing with “true lateral” and        anterior/posterior fluoroscope images of the knee while in        extension:        -   a. Ideally, the prosthesis should be within 1 mm of the            anterior/posterior boundaries of the tibial plateau and            superimposed over the medial boundary.    -   9. Remove trial component and flush joint with saline.    -   10. Insert the appropriate prosthesis.    -   11. Confirm proper placement and sizing with fluoroscopic images        as with trial component.    -   12. Maintain leg in extension and close wound after insertion of        a Hemovac drain.    -   13. Place leg in immobilizer prior to patient transfer.

While embodiments of the invention have been illustrated and described,it is not intended that these embodiments illustrate and describe allpossible forms of the invention. The words used in the specification arewords of description rather than limitation, and it is understood thatvarious changes may be made without departing from the spirit and scopeof the invention.

1. A knee prosthesis for implantation into a knee joint compartmentbetween a femoral condyle and its corresponding tibial plateau, theprosthesis comprising: a body having a generally elliptical shape inplan and having a material thickness at a part center (CSO) thereof, thebody including a pair of opposed surfaces wherein one of the surfaces isat least partially concave, the prosthesis free of any means of fixationwithin the knee joint compartment and configured to be movable withinthe knee joint during knee articulation, the body having an exteriorportion and an interior portion, wherein the exterior portionencapsulates the interior portion and the exterior portion isconstructed from a higher modulus material than the interior portion. 2.The prosthesis according to claim 1, wherein the interior portion issubstantially hollow.
 3. The prosthesis according to claim 1, whereinthe interior portion is constructed from a material selected from thegroup consisting of hydrogels, reinforced and non-reinforced elastomericpolymers, and viscous-elastic materials.
 4. The prosthesis according toclaim 1, wherein the exterior portion is constructed of a biocompatiblematerial selected from the group consisting of ceramics, metals, metalalloys, hydrogels, reinforced and non-reinforced thermoset orthermoplastic polymers, or composites thereof.
 5. The prosthesisaccording to claim 1, wherein one of the exterior portion and theinterior portion is constructed at least partially of a material capableof containing living cells.
 6. The prosthesis according to claim 1,further comprising a surface coating applied to the exterior portion. 7.The prosthesis according to claim 1, wherein at least one of theexterior portion and interior portion includes an active materialassociated therewith.
 8. The prosthesis according to claim 1, furtherincluding a locking mechanism disposed in the interior portion andarranged to connect the pair of opposed surfaces.
 9. The prosthesisaccording to claim 8, wherein the locking mechanism includes a firstmating component affixed to a first one of the pair of opposed surfacesand a second mating component affixed to a second one of the pair ofopposed surfaces and arranged to be securely received by the firstmating component, wherein the first and second mating components canmove relative to one another.
 10. The prosthesis according to claim 1,wherein the pair of opposed surfaces includes a top surface that isgenerally concave.
 11. The prosthesis according to claim 1, wherein thepair of opposed surfaces includes a bottom surface that is generallyflat.
 12. The prosthesis according to claim 1, wherein the top surfaceand the bottom surface are contoured such that the prosthesis isself-centering within the knee joint compartment.
 13. The prosthesisaccording to claim 1, further including a peripheral edge extendingbetween the pair of opposed surfaces and having a first side, a secondside opposite the first side, a first end, and a second end opposite thefirst end.
 14. The prosthesis according to claim 13, wherein a firstdimension D is defined by the first end and the second end, and a seconddimension F is defined by the first side and the second side, whereinthe dimension F is from about 0.25D to about 1.5D.
 15. The prosthesisaccording to claim 13, wherein a section across the body from the firstend to the second end generally has the shape of a negative meniscus.16. The prosthesis according to claim 13, wherein a section across thebody from the first side to the second side has a thickness at aperiphery of the first side which is larger on average than a thicknessat a periphery of the second side.
 17. The prosthesis according to claim1, wherein outside edges along a periphery of the body are rounded. 18.The prosthesis according to claim 1, wherein a periphery of the body ison average of greater thickness than a central portion of the body. 19.A method for implantation of a knee prosthesis into a knee jointcompartment between a femoral condyle and its corresponding tibialplateau, the method comprising: providing a knee prosthesis including abody having a generally elliptical shape in plan and having a materialthickness at a part center (CSO) thereof, the body including a pair ofopposed surfaces wherein one of the surfaces is at least partiallyconcave, the prosthesis free of any means of fixation within the kneejoint compartment and configured to be movable within the knee jointduring knee articulation, the body having an exterior portion and aninterior portion, wherein the exterior portion encapsulates the interiorportion and the exterior portion is constructed from a higher modulusmaterial than the interior portion; surgically exposing the knee jointcompartment; and inserting the prosthesis into the knee jointcompartment.
 20. The method according to claim 19, further comprisingproviding an active material associated with the body.
 21. The methodaccording to claim 19, further comprising altering the condition oftissue located in the knee joint compartment or adjacent thereto. 22.The method according to claim 19, further comprising determining a sizeand shape of the prosthesis required by examination of the knee joint.23. The method according to claim 22, wherein the examination includesone or more of X-ray imaging and MRI imaging.
 24. The method accordingto claim 22, further comprising selecting the prosthesis from a libraryof prostheses of standard shapes and sizes.
 25. The method according toclaim 22, further comprising generating a custom prosthesis whose sizeand shape are at least partially based on the examination of the kneejoint.
 26. The method according to claim 19, further comprisingpre-loading the prosthesis during implantation.
 27. A method forcorrecting misalignment in an axis of rotation of a knee joint, themethod comprising: providing a knee prosthesis including a body having agenerally elliptical shape in plan and having a material thickness at apart (CSO) thereof, the body including a pair of opposed surfaceswherein one of the surfaces is at least partially concave, theprosthesis free of any means of fixation within the knee jointcompartment and configured to be movable within the knee joint duringknee articulation, the body having an exterior portion and an interiorportion, wherein the exterior portion encapsulates the interior portionand the exterior portion is constructed from a higher modulus materialthan the interior portion; surgically exposing the knee joint; andinserting the prosthesis into the knee joint to at least partiallycorrect the misalignment of the axis of rotation.
 28. The methodaccording to claim 27, wherein inserting the prosthesis includesinserting the prosthesis into a medial compartment of the knee joint andmoving the axis to a less varus condition.
 29. The method according toclaim 27, wherein inserting the prosthesis includes inserting theprosthesis into a lateral compartment of the knee joint and moving theaxis to a less valgus condition.